Why most of UK people like to chew words?
I mean for example look at this video and the way the girl speaks - youtube.com/watch?v=SfSmWMNOw2M
I know my English is not perfect but I can barely recognize her speech. Only if I listen 2+ times I can discern what she is actually saying.

that because the way u pronounce word is probably different to us and you are Brazilian so no wonder you pronounce word different.
also you are talking about 1 Aussie and if hes from New South Wales or Queensland then i understand if his English is a bit more wacky not being mean to those Aussies but they just speak a bit different to the south of Australia.
I'm from Victoria which is a state in Australia and a lot of us that live in city's speak a bit different to the ones that live out on farms.

Until the 1930s communication between towns was quite limited and so dialects developed. These have been jealously preserved as a means of retaining an identity. The other English speaking countries were developed by a mixed group of people from Britain, so no dialect prevailed and words had to be more general for all to understand.
England is where English came from, so that is where there has been the most time to generate new dialects.
When people moved to e.g. Australia, many of the immigrants were from a certain part of the country, and they relatively quickly colonized the place. Same goes with the introduction of English in other parts of the world, which is why it is more noticeable what town someone in Britain is from.
Second, regional accents are not unique to the UK. The simple fact is that until a couple of hundred years ago people all over the world lived in fairly self contained communities where few people travelled more than ten miles or so from cradle to grave. Those that did travel were lost to their birth communities as sure as if they emigrated abroad. As a result language and pronouciation evolved independently from town to town, and region to region.
The thing that changed this was the invention of the train, which changed the world forever. People could move between communities at will, and language, even, in the UK has drifted to a more common form. And television and radio have accelerated this.
However each generation stills learns to talk from the previous generation of their family and that legacy means that these historical differences will persist for generations.
The US also has wide accent differences between states, but the mobility of citizens in the present day and historically probably makes this less significant.

It's hard to explain in few words but basically there are different accents everywhere but because we were the ones who created the language before their was mass communication and options to travel to other towns, areas developed their own accents which is evelutionary, because by the time it spread to other areas of the world they had travel and communication availability it didn't stagnate as people were forced to talk to each other.

Copying my comment from above for visibility.
Until the 1930s communication between towns was quite limited and so dialects developed. These have been jealously preserved as a means of retaining an identity. The other English speaking countries were developed by a mixed group of people from Britain, so no dialect prevailed and words had to be more general for all to understand.
England is where English came from, so that is where there has been the most time to generate new dialects.
When people moved to e.g. Australia, many of the immigrants were from a certain part of the country, and they relatively quickly colonized the place. Same goes with the introduction of English in other parts of the world, which is why it is more noticeable what town someone in Britain is from.
Second, regional accents are not unique to the UK. The simple fact is that until a couple of hundred years ago people all over the world lived in fairly self contained communities where few people travelled more than ten miles or so from cradle to grave. Those that did travel were lost to their birth communities as sure as if they emigrated abroad. As a result language and pronouciation evolved independently from town to town, and region to region.
The thing that changed this was the invention of the train, which changed the world forever. People could move between communities at will, and language, even, in the UK has drifted to a more common form. And television and radio have accelerated this.
However each generation stills learns to talk from the previous generation of their family and that legacy means that these historical differences will persist for generations.
The US also has wide accent differences between states, but the mobility of citizens in the present day and historically probably makes this less significant.

Idk, the UK is so small, but you have a ton of different accents from each region. We basically don't have any accents in Russia (besides the southern, where it is influenced by Ukrainian). I live like 7000 kilometers away from the central Russia, but I speak the same way as them.

You don't, there are differences in pronunciation between for example russian in Tula and russian in Yekaterinburg, but 98% of people don't hear any of them.
And on Caucasus you will barely find someone who speaks proper russian